In a fixed network, the availability and the quality of transmission paths change dynamically. These changes are even more frequent in networks consisting of mobile stations in a hostile environment. An effective network should know at all times what paths exist and which are the best among them, and switch rapidly to new paths as the situation changes.
When a bit stream is sent over any optical link, it may undergo degradation due to attenuation, dispersion, noise, and jitter, among other things. As the shape and amplitude of the bits change, the receiver's ability to reliably distinguish 1's from 0's is also reduced. In current technology, the degree of degradation is typically measured by sending a very long pseudo-random bit stream over a link and comparing the receiver's best guess for each bit with the original signal. At a typical phone line bit error rate (BER) of 10−9, a billion bits must be received on the average before a single error is detected, and usually at least 100 errors are required (1011 bits) for the measurement to be statistically significant. For data links, BER's of 10−12 are not uncommon. To receive, then, the requisite 1014 bits, about 40 minutes are required at the fastest bit rate (40 Gb/s) and even longer for more typical links. Alternatively, one may measure the eye diagram, in which one collects many bits (thousands), converts them to an electronic signal, and superposes the various bits on an oscilloscope. Then sophisticated electronics examine a window inside the eye and look for transgressions of the signal into the opening of the eye. The advantage of the eye diagram technique is that attenuation, dispersion, jitter, and noise can all be determined from the shape of the eye, but it may require 20 to 30 seconds at the fastest bit rate, and the information has to be processed by a person, and then acted on.